Fix how default data_dir argument is set for the function install() per #43 and #44.
Roxygen created a difference on this line and I am adding it for convience sake.
To ensure windows compatibility the path of the data directory for install must always be specified in the call to the retriever.
Merge pull request #45 from dmcglinn/install_fix

Implement a fix for the function install() so that it works on Windows OS